Hydraulic cements are cements that set and develop compressive strength due to a hydration reaction, and thus can be set under water. As such, hydraulic cements are often used for cementing pipes or casings within a wellbore of a subterranean formation as well as other purposes, such as squeeze cementing. In the oil and gas industry, successful cementing of well pipe and casing during oil and gas well completion requires cement slurries having several important properties. The cement slurry must have a pumpable viscosity, fluid loss control, minimized settling of particles and the ability to set within a practical time.
In a typical completion operation, the cement slurry is pumped down the inside of the pipe or casing and back up the outside of the pipe or casing through the annular space. This seals the subterranean zones in the formation and supports the casing. Under normal conditions, hydraulic cements, such as Portland cement, quickly develop compressive strength upon introduction to a subterranean formation, typically within 48 hours from introduction. As time progresses, the cement develops greater strength while hydration continues.
It is common to use a set retarder with the hydraulic cement in order to delay the set time of the cement composition. Such set retarders are particularly useful when the cement composition is exposed to high subterranean temperatures. In addition to being capable of delaying the set time of the cement composition, the set retarder also functions to extend the time the cement composition remains pumpable after the cement composition is mixed and before it is placed into the desired location.
In use, many of the set retarders of the prior art exhibit unpredictable retardation of the set time of the cement composition especially at elevated temperatures. For instance, lignosulphonates are often used with borate retarder intensifiers to retard Portland cement in oil wells at temperatures in excess of 350° F. Such set retarder compositions, however, are only somewhat effective as downhole temperatures increase to 500° F.
A need therefore exists for the development of cement compositions containing a set retarder (preferably which does not require an intensifier) and which is effective at downhole temperatures in excess of 500° F.